<!DOCTYPE html>
<html lang="zh">
<head>
    <meta charset="UTF-8">
    <title>01-id选择器</title>
    <style>

        #the-main-content-happy {
            color: red;
        }

    </style>
</head>
<body>

<p>我是p1</p>
<p>我是p2</p>
<p id="the-main-content-happy">我是p3</p>
<p id="main_title_happy">我是p4</p>
<p id="theMainTitleHappy">我是p4</p>

<!-- 保证代码的可读性 -->

<div id="title">我是div</div>

</body>
</html>